Because of the commercial and regulatory demands to reduce vehicle emissions, a possible viable alternative, which many automakers are researching, is the use of hydrogen as a fuel source for fuel cells or combustion engines. There are several technical obstacles to achieving an acceptable hydrogen-based powering system for vehicles. Some of these obstacles include developing an economical process for the manufacture of hydrogen, constructing a refueling infrastructure, as well as ensuring a safe and effective on-board hydrogen storage system.
Most fuel cell and combustion engine designs require the delivery of hydrogen in the form of a gas, but current hydrogen storage cylinders do not have the volume capacity that would allow for a reasonable driving range. In order to fill the cylinders with enough hydrogen gas to achieve a reasonable driving range, cylinders would need to be developed that can withstand high pressures, e.g. >10,000 psi. The hydrogen gas delivery system of the vehicle would then require careful design and selection of materials that can safely maintain these excessive pressures, resulting in increased costs of the vehicle design. Therefore, a vehicle which stores hydrogen in a gaseous state is unlikely to provide an acceptable solution.
An alternative to using highly-pressurized vessels to hold hydrogen gas is the use of solid metal hydrides. Instead of filling a vessel with pressurized and combustible hydrogen gas, the vessels are filled with metal hydride powder. The solid offers a much safer form. Upon heating the metal hydride, a reaction occurs causing the release of hydrogen gas. When refilling the vessel, the dehydrided material will absorb the hydrogen gas and return to a hydrided state. The typical pressures for these hydrogen storage systems is much lower, around 250 psi. Also, a vessel containing metal hydride is able to store more hydrogen than a cylinder containing only hydrogen gas. Thus, metal hydride systems offer a safer and more-cost effective alternative to pressurized hydrogen gas cylinders.
In recent years, complex metal hydrides have been widely studied for hydrogen storage applications because of their relatively high hydrogen-holding capacities. Systems of interest include alanates, borohydrides, and amides. However, various scientific and technological barriers need to be overcome to optimize the potential of these materials. Problems such as high desorption temperatures, slow kinetics, poor reversibility due to loss of ammonia or diborane, and the formation of stable intermediates have been identified with many of these systems. Some mixtures of these compounds have yielded improvements. Examples of binary mixtures that have been studied include LiNH2/MgH2, Mg(NH2)2/2LiH, LiBH4/CaH2, LiBH4/MgH2, Mg(BH4)2/Ca(BH4)2, LiBH4/LiNH2, LiNH2/LiH, and MgH2/LiH.
The U.S. Department of Energy Metal Hydride Center of Excellence (MHCoE) has described the lithium amide/magnesium hydride (2LiNH2/MgH2) system as an important “near term” system for hydrogen storage that would be a good candidate for engineering subsystem testing. This is because of its good long-term cycling behavior and higher hydrogen capacity than other metal hydrides currently investigated for their hydrogen storage capacity, such as sodium aluminum hydride. The mechanism of its hydrogen release and the general reaction scheme for the 2LiNH2/MgH2 system proceed according to the following equations:MgH2+2LiNH2→Mg(NH2)2+2LiH  (1)Mg(NH2)2+2LiHLi2Mg(NH)2+2H2  (2).
Recently, more work has been focused on lowering the desorption temperature and improving the kinetics, i.e. the rate of hydrogen desorption, of the 2LiNH2/MgH2 system with effective catalysts. Several additives such as NaOH, V, V2O5, VCl3, Si, and Al have been tried. Perhaps the most effective catalyst to date has been potassium hydride (KH). However, there is still a need to identify catalysts that will further improve the kinetics and lower the desorption temperatures for these hydrogen storing alloys.